Difficulty in rising from and returning to a chair is common in the elderly and is associated with an increased risk of hip fractures and falls. Comprehensive geriatric assessment measures, which include rising from and returning to a chair, are generally descriptive, qualitative, and rarely performance-oriented. We propose a more quantitative approach, using biomechanical analyses, to assess rising from and returning to a chair in the elderly. Measurements will quantify mild-moderate dysfunction and functional changes over time, and will help clarify relationships among aging, musculoskeletal impairment, and disability. Moreover, quantitative data will aid in establishing goals for strength and balance therapies, developing body movement strategies, and modifying environments to maintain or improve chair mobility in the elderly. Using an optoelectronic motion acquisition system and seat and foot force plates, we will analyze rising from a chair, achieving standing balance, and returning to a seated position. Performance of healthy elderly subjects in these tasks will be considered in two analyses: 1) in comparison to young and middle-aged adults; 2) in comparison to elderly adults who have difficulty in rising from a chair due to musculoskeletal impairments. The conditions for rising from the chair will be altered, specifically regarding the use of armrests, height of the seat, and foot support surface area and location. We will then use the motion and force data to simulate these tasks in a computer model, that will compute the joint torque strengths required. Finally, we will compare body segment motions and computed joint torques used in these tasks with clinical measures of joint range of motion and muscular strength, respectively. The overall goal is to quantitatively determine the importance of strength, joint range of motion, and postural stability in performing these tasks, particularly in elderly subjects with musculoskeletal impairment of the knee.